Alzheimer's disease (AD) is by far the most frequent disease causing dementia, and AD-like mechanisms like amyloid deposition are also implicated in the two next most common causes, vascular (VaD) and dementia with Lewy bodies (DLB). AD runs a protracted course from the time of diagnosis and is detrimental also to the health of caregivers [1, 2]. In a system of managed care, AD also is a severe burden to the public health system [3], the economic impact is by some estimates already larger than that of cancer, stroke and heart disease [4, 5]. AD prevalence increases from 10% above 65 years of age, to 50% among those above 85 [6, 7] and may quadruple by 2050 due to incidence rates that increase exponentially with age [8] and a larger number of elderly [9].
Today, we appear to be on the verge of therapy against Alzheimer's disease (AD) progression [10]. This will dramatically increase the importance of precise early diagnosis, and will shift the research focus towards an understanding of the process of AD disease induction.
The characterization of patients with Mild Cognitive Impairment (MCI) [11] and recent proposed research criteria for AD [12] help identify a patient group with increased risk for progressive dementia [13]. It is known that AD initiation and progression is linked to central nervous system (CNS) APP (amyloid precursor protein) production; the metabolism of APP to Aβ42 protein by β- and γ-secretase; and the deposition of the Aβ42 protein in amyloid plaques [14, 15]. Tau aggregation, microtubule disassembly and neurofibrillary degeneration follows [10, 16, 17], possibly as a result of interaction with Aβ42 [18-20]. Disease development is strongly influenced by genetic disposition [21, 22], but probably also by epigenetic and acquired risk factors like cerebrovascular disease [23] and proteomic [24] and immunological [25] mechanisms. The concentration of Aβ42 in cerebro-spinal fluid (CSF) reflects CNS parenchymal levels and increases with age above 50 [26]. However, the concentration of Aβ42 in CSF is reduced in patients that develop AD [27, 28] probably due to the deposition in amyloid plaques [29].
The length of the time span from AD initiation to development of dementia in individual patients is unknown. Evidence from autopsies performed on patients that have died of unrelated causes suggests that limited early stage AD-like damage may occur decades earlier [30], but the natural evolution of these lesions is unknown. An extended preclinical phase from disease initiation to clinical dementia may give a large therapeutic time-window.
Dementia is preceded by mild cognitive impairment (MCI) [11, 13, 31]. At this stage, some patients are at increased risk of progression to dementia (annual rate of conversion of 6-25%, [13]), though others may have a condition limited to MCI for a number of years. Imaging evidence indicates that a subgroup of patients with MCI has brain amyloid deposition [32-34]. This evidence supports results from studies with CFS markers, where low levels of the amyloid precursor CSF Aβ42 linked to cerebral amyloid deposition [18, 29] tend to be found in MCI subgroups that subsequently progress to Alzheimer dementia. Many of these patients satisfy new AD research criteria [12]. As described above, neuropsychology, CSF proteomics and neuroimaging have contributed to increased understanding of MCI. Induction of disease occurs earlier, and a period of latency may overlap with the MCI stage, as the disease becomes detectable.
However, there is still no reliable means for detecting the early stages of AD, even though these early periods may turn out to be clinically valuable for implementation of disease modifying therapies [35]. This is significant because, at later stages, the widespread damage caused by AD is most likely irreversible [16]. Therefore, early detection of AD is important to its effective treatment.
Fiala M. et al. [39] reports on a study in which monocytes (CD68 positive cells) obtained from blood samples of AD patients and control individuals were differentiated into macrophages and then exposed to Aβ protein in vitro. The Aβ protein was conjugated with a visible marker and the cells examined by fluorescence or confocal microscopy in order to determine uptake of Aβ protein. The study indicates that the macrophages derived from AD patients were inefficient in Aβ protein phagocytosis compared with the control. However, the approach taken in this study is rather laborious and it is not clear at what stage in the development of AD this technique would provide a positive result.
Other pathological conditions of the central nervous system which are characterised by the presence of fragments of a marker protein the brain include Parkinson's Disease, Multiple Sclerosis, Fronto Temporal Dementia and Amyotrophic Lateral Sclerosis. In each case, early diagnosis of the condition is desirable.
Therefore, the present invention seeks to alleviate one or more of the above problems and provide an improved method of detecting the presence or monitoring the severity of a condition, characterised by the presence of fragments of a marker protein in the brain of a patient.